Well I've been playing around with the intake on my 2000 Civic DX, and found some interesting results.
The WAI I originally made for the car which consisted of a 3" dryer vent tube fastened to the intake filter lid and tucked under the exhaust header wound up looking great in design, but turned out to be a little TOO much heat for the car to enjoy. I wound up getting a lifetime low of 40mpg with it attached. However when I took off the WAI, and just had the stock intake out of the car (the tube attached to the intake lid, and the giant intake box looking thing?) and just ran the filter with the lid in place, I got higher mileage than I did with the stock intake intact. It's been getting great mileage ever since, too!
I think the way it sits it pulls in
warmer air from the top of the engine bay, but isn't pulling in so much heated air that it kills it. I'm going to be playing around to try to find a sweet spot where I can get the hottest air in the engine to increase my mpg's.
I just wanted to post this up there for others who may have overlooked their intakes, to take a look at it. See if you can remove some of the stock intake to get more/warmer air into the engine. In my car I only had to remove 2 bolts, and voila! better mileage!
(I'll try to get some pics to better explain what I mean
)